I’ve used the editor from http://www.paulspoerry.com/code/post-editor-for-google-plus/ for a while, though it only works if you’re a Chrome user, (for some reason, it doesn’t always show for me – sometimes I have to click a * to trigger it into action, but that could be me) however, I’ve just seen http://www.thepluseditor.com/ as well – which works on other (all?) browsers – but involves cutting & pasting from it to G+
Post Editor for Google+: simple formatting assistance for Google+